docs: def answer(self, allow, server_url=None, identity=None, claimed_id=None):
          """Respond to this request.
  
          @param allow: Allow this user to claim this identity, and allow the
              consumer to have this information?
          @type allow: bool
  
          @param server_url: DEPRECATED.  Passing C{op_endpoint} to the
              L{Server} constructor makes this optional.
  
              When an OpenID 1.x immediate mode request does not succeed,
              it gets back a URL where the request may be carried out
              in a not-so-immediate fashion.  Pass my URL in here (the
              fully qualified address of this server's endpoint, i.e.
              C{http://example.com/server}), and I will use it as a base for the
              URL for a new request.
  
              Optional for requests where C{CheckIDRequest.immediate} is C{False}
              or C{allow} is C{True}.
  
          @type server_url: str
  
          @param identity: The OP-local identifier to answer with.  Only for use
              when the relying party requested identifier selection.
          @type identity: str or None
  
          @param claimed_id: The claimed identifier to answer with, for use
              with identifier selection in the case where the claimed identifier
              and the OP-local identifier differ, i.e. when the claimed_id uses
              delegation.
  
              If C{identity} is provided but this is not, C{claimed_id} will
              default to the value of C{identity}.  When answering requests
              that did not ask for identifier selection, the response
              C{claimed_id} will default to that of the request.
  
              This parameter is new in OpenID 2.0.
          @type claimed_id: str or None
  
          @returntype: L{OpenIDResponse}
  
          @change: Version 2.0 deprecates C{server_url} and adds C{claimed_id}.
  
          @raises NoReturnError: when I do not have a return_to.
          """
          assert self.message is not None
  
          if not self.return_to:
              raise NoReturnToError
  
          if not server_url:
              if not self.message.isOpenID1() and not self.op_endpoint:
                  # In other words, that warning I raised in Server.__init__?
                  # You should pay attention to it now.
                  raise RuntimeError("%s should be constructed with op_endpoint "
                                     "to respond to OpenID 2.0 messages." %
                                     (self,))
              server_url = self.op_endpoint
  
          if allow:
              mode = 'id_res'
          elif self.message.isOpenID1():
               if self.immediate:
                   mode = 'id_res'
               else:
                   mode = 'cancel'
          else:
              if self.immediate:
                  mode = 'setup_needed'
              else:
                  mode = 'cancel'
  
          response = OpenIDResponse(self)
  
          if claimed_id and self.message.isOpenID1():
              namespace = self.message.getOpenIDNamespace()
              raise VersionError("claimed_id is new in OpenID 2.0 and not "
                                 "available for %s" % (namespace,))
  
          if allow:
              if self.identity == IDENTIFIER_SELECT:
                  if not identity:
                      raise ValueError(
                          "This request uses IdP-driven identifier selection."
                          "You must supply an identifier in the response.")
                  response_identity = identity
                  response_claimed_id = claimed_id or identity
  
              elif self.identity:
                  if identity and (self.identity != identity):
                      normalized_request_identity = urinorm(self.identity)
                      normalized_answer_identity = urinorm(identity)
  
                      if (normalized_request_identity !=
                          normalized_answer_identity):
                          raise ValueError(
                              "Request was for identity %r, cannot reply "
                              "with identity %r" % (self.identity, identity))
  
                  # The "identity" value in the response shall always be
                  # the same as that in the request, otherwise the RP is
                  # likely to not validate the response.
                  response_identity = self.identity
                  response_claimed_id = self.claimed_id
              else:
                  if identity:
                      raise ValueError(
                          "This request specified no identity and you "
                          "supplied %r" % (identity,))
                  response_identity = None
  
              if self.message.isOpenID1() and response_identity is None:
                  raise ValueError(
                      "Request was an OpenID 1 request, so response must "
                      "include an identifier."
                      )
  
              response.fields.updateArgs(OPENID_NS, {
                  'mode': mode,
                  'return_to': self.return_to,
                  'response_nonce': mkNonce(),
                  })
  
              if server_url:
                  response.fields.setArg(OPENID_NS, 'op_endpoint', server_url)
  
              if response_identity is not None:
                  response.fields.setArg(
                      OPENID_NS, 'identity', response_identity)
                  if self.message.isOpenID2():
                      response.fields.setArg(
                          OPENID_NS, 'claimed_id', response_claimed_id)
          else:
              response.fields.setArg(OPENID_NS, 'mode', mode)
              if self.immediate:
                  if self.message.isOpenID1() and not server_url:
                      raise ValueError("setup_url is required for allow=False "
                                       "in OpenID 1.x immediate mode.")
                  # Make a new request just like me, but with immediate=False.
                  setup_request = self.__class__(
                      self.identity, self.return_to, self.trust_root,
                      immediate=False, assoc_handle=self.assoc_handle,
                      op_endpoint=self.op_endpoint, claimed_id=self.claimed_id)
  
                  # XXX: This API is weird.
                  setup_request.message = self.message
  
                  setup_url = setup_request.encodeToURL(server_url)
                  response.fields.setArg(OPENID_NS, 'user_setup_url', setup_url)
  
          return response